Motor configurations

AN235

4/23

Doc ID 1679 Rev 2

1

Motor configurations

From a circuit designer's point of view stepper motors can be divided into two basic types:

unipolar and bipolar. A stepper motor moves one step when the direction of current flow in

the field coil(s) changes, reversing the magnetic field of the stator poles. The difference

between unipolar and bipolar motors lies in the way that this reversal of the magnetic field is

achieved (Figure 1). The bipolar motor has one coil per phase and needs two changeover

switches, or a full-bridge, for each phase. The switches reverse the direction of current flow

in the coil. The unipolar motor has a center tapped coil on each phase and needs one

changeover switch, or two transistors to ground, for each phase. The switches select which

half of the coil current flows through.

Figure 1.

Stepper motor configuration

The advantage of the bipolar circuit is that there is only one winding, with a good bulk factor

(low winding resistance). The main disadvantage is the more complex drive circuit needing

the two changeover switches for each phase. This is implemented as a full H-bridge for each

phase and requires more transistors that the unipolar configuration.

The unipolar circuit needs only one changeover switch, implemented as two transistors to

ground, for each phase. Its enormous disadvantage is, however, that a double bifilar winding

is required. This means that at a specific bulk factor the wire is thinner and the resistance is

much higher. The problems involved are going to be discussed in this application note.

Unipolar motors are still popular today for low performance applications because the drive

circuit is simpler when implemented with discrete devices. However, with the integrated

circuits available today, bipolar motors can be driver with no more components than the

unipolar motors.

Figure 2 compares integrated unipolar and bipolar driver ICs. The unipolar driver integrates

the four transistors to ground and the four freewheeling diodes. The bipolar driver integrates

two full H-bridges and the 8 freewheeling diodes.

A: Bipolar motor configuration

B: Unipolar motor configuration

AM16499v1